Description The only cephalopod family including species with external true shells. The shells are large, light in weight and resemble a round-bottomed boat. They are distinguished, above all, by being partitioned or chambered internally, the partitions being linked by a tube known as siphuncle. The chambers are filled with a gas allowing the shell to float; the animal controls its buoyancy by introducing water into or removing it from the chambers. Two genera, six or seven species, all from the Indo-Pacific. [details]